The invention relates to a process for preparing non-porous membrane layers. Many processes are known for the preparation of membrane layers such as solvent spreading, dip-coating, plasma polymerization and spraying of membrane-forming compounds on porous or non-porous substrates.
A problem which arises when porous substrates are to be coated with a non-porous i.e. dense membrane layer in order to produce pinhole-free composite membranes is the preparation of a non-porous layer with sufficient strength to bridge the pores in the substrate without blocking them, i.e. without substantial penetration of the non-porous layer into the pores. Blocked substrate pores would result in an undesirable decrease in permeability of the composite membranes for the fluid compounds which have to be separated, whereas the presence of substrate pores which are not covered by the non-porous membrane which would result in undesired transfer of fluid compounds through the composite membrane i.e. a decrease in selectivity.